1. Field of the Invention
The invention relates to an improved fuel injection system for internal combustion engines.
2. Description of the Prior Art
For introducing fuel into direct-injection Diesel engines, both stroke- and pressure-controlled fuel injection systems are known. For better comprehension of the description and claims, several terms will first be explained: The fuel injection according to the invention can be done by either stroke or pressure control. Within the scope of the invention, a stroke-controlled fuel injection is understood to mean that the opening and closing of the injection opening is accomplished with the aid of a displaceable valve member because of the hydraulic cooperation of the pressures in a nozzle chamber and in a control chamber. A pressure reduction within the control chamber causes a stroke of the valve member. Alternatively, the deflection of the valve member can be accomplished by a final control element (actuator). In a pressure-controlled fuel injection according to the invention, the valve member is moved counter to the action of a closing force (spring) by the fuel pressure prevailing in the nozzle chamber of an injector, so that the injection opening is opened for an injection of the fuel from the nozzle chamber into the cylinder. The pressure at which fuel emerges from the nozzle chamber into a cylinder of an internal combustion engine is called the injection pressure, while a system pressure is understood to mean the pressure at which fuel is available or is kept on hand within the fuel injection system. Fuel metering means furnishing a defined fuel quantity for injection. The term or leak fuel, is understood to mean a quantity of fuel that occurs in operation of the fuel injection system (such as a reference leakage) and is not used for injection and is pumped back to the fuel tank. This leak fuel can have a standing pressure, after which the fuel is depressurized to the pressure level of the fuel tank.
It is also known to use a pressure booster, in order to have not only the rail pressure but a further, different injection pressure available. The use of a separate working medium (such as hydraulic oil) for actuating the pressure booster has the disadvantage that it is no longer possible to use the rail pressure as the injection pressure.
For embodying a flexible fuel injection system which uses a separate working medium (hydraulic oil) for actuating the pressure booster, a fuel injection system is proposed according to the invention.
To enhance the flexibility of a fuel injection system, besides the fuel pressure of the hydraulic oil-actuated pressure booster (first system pressure), a further, second (low) fuel system pressure is generated, which can be used for injection. The second system pressure is stored as needed in a pressure reservoir and is applied constantly to the injector. A flexible shaping of the injection course and multiple injection can be attained. For generating pressure, a separate high-pressure pump can be used. However, it is also possible for the fuel pressure to be generated with a central pressure booster. Advantageously, the second system pressure can also be furnished by means of storage of a portion of the fuel compressed by the pressure booster.
If the fuel pressure is selected to be higher than the oil pressure in the pressure reservoir, then a hydraulic restoring force acts on a piston of the local pressure booster. The requisite restoring spring can thus be reduced in size or even omitted. This has a major advantage in terms of installation space, which is important especially for integrating the pressure booster with the injector.